msm_fb: display: remove background invert alpha option for alpha drop pipe

When the blended background pipe has dropped alpha and it is going to
be blended with foreground pipe on the next stage,
MDP4_BLEND_BG_ALPHA_FG_CONST is taken. Foreground alpha constant is
0xFF, so we need to use this alpha value to show background, instead
of its inverted value.

Change-Id: I1fe0572f9bb54db6280bcc44117a21b8c0d8b07a
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>